Most, if not all, musical instruments produce varying degrees of harmonic change with time as they are played. Some instruments exhibit this change during the onset of tone production and then settle down harmonically to a "steady state" condition. Examples of such instruments would be horns, bowed strings, and organ pipes. There are other instruments where the harmonic change occurs throughout the audible sound production. Examples of these would be plucked strings, bells and the piano. In synthesizing the sounds of musical instruments electronically, harmonic changes with time are an important contribution to realism.
In presently manufactured electronic musical instruments, harmonic variations with time have been implemented with varying degrees of success. One such approach is described in U.S. Pat. No. 4,184,403, assigned to the assignee of the present invention, wherein waveform generation is accomplished by successively reading out amplitude samples a waveform from a memory. The harmonic content of the voice or audible tone being generated is caused to change by reading from multiple memories singly but in sequence where each memory contains a slightly different harmonic content. The major deficiency in reproducing audible tones in the manner just described is the fact that under certain circumstances, such as a long, gradual decay, it becomes apparent to the listener that a sequence of discrete harmonic structures is being generated as opposed to a smooth or gradually changing harmonic sequence.
It is therefore an object of the present invention to eliminate the stepped or discrete sequence of harmonic structures as noted by a listener of the reproduced audible tone by providing interpolation between the discrete harmonic structures stored in the memory.
It is another object of the present invention to be able to interpolate between the discrete harmonic structures of the tone or note without introducing quantizing errors which may cause distortion to the listener.
It is still a further object of the present invention to control the rate at which the interpolation between the discrete harmonic structures proceeds in order to adjust such rate to one which most closely approaches the actual transient harmonic of the selected voice.
Other objects will appear hereinafter.